This is a single-arm, open-label, multicenter study if Iodine-131 Anti-B1 Antibody for the treatment of patients with non-Hodgkin's lymphoma (NHL) who previously responded (PR, CCR, or CR) with a duration of response of at least 3 months to Iodine-131 Anti-B1 Antibody therapy. Patients will undergo two phases of study. In the first phase, termed the "dosimetric dose," patients will receive an infusion of unlabeled Anti-B1 Antibody (450 mg) over 70 minutes (including a 10-minute flush) immediately followed by a 30-minute infusion (including a 10- minute flush) of Anti-B1 Antibody (35 mg) which has been radiolabeled with 5 mCi of Iodine-131. Whole body gamma-camera scans will be obtained on day 0, day 2, 3, or 4, and day 6 or 7 following the dosimetric dose. Using the dosimetric data from the three imaging timepoints, a patient-specific dose of Iodine-131 to deliver the desired total body dose of radiotherapy will be calculated. In the second phase, termed the "therapeutic dose", patients will receive a 70-minute infusion (including a 10-minute flush) of unlabeled Anti-B1 Antibody (450 mg) immediately followed by a 30-minute infusion (including a 10 minute flush) of 35 mg Anti-B1 Antibody labeled with the patient-specific dose of Iodine-131 to deliver the desired whole body dose of radiation. The whole body radiation dose will be 75cGy for patients with a baseline platelet count equal to/greater than 150,000 cells/mm who have not undergone prior bone marrow or stem cell transplant. The whole body radiation dose will be 65 cGy for patients with a baseline platelet count of 100,001 - 149,999 cells/mm who have not undergone prior bone marrow or stem cell transplant. The whole body radiation dose will be 55 cGy for patients with a baseline platelet =150,000 cells/mm who have previously undergone bone marrow or stem cell transplant. The whole body radiation dose will be 45 cGy for patients who have a baseline platelet count of 100,001 - 149,999 cells/mm and have previously undergone bone marrow or stem cell transplant. For excessively obese patients (patients weighing more than 137% of their lean body mass), the calculations to determine Iodine-131 Anti-B1 Antibody activity to administer will be performed using an upper limit of mass (maximum effective mass) based upon height and gender. Patients will be treated with either saturated solution potassium iodide (SSKI), Lugol's solution, or potassium iodide tablets starting at least 24 hours prior to the first infusion of the Iodine-131 Anti-B1 Antibody (i.e., the dosimetric dose) and continuing for 14 days following the last infusion of Iodine-131 Anti-B1 Antibody (i.e., therapeutic dose). The endpoints of the study are to determine the response rate, complete response rate, duration of response, time-to-progression, time-to-treatment failure, safety, and survival following Iodine-131Anti-B1 Antibody therapy in patients with NHL who have previously responded (PR, CCR, CR) with a duration of response of at least 3 months to Iodine-131 Anti-B1 Antibody therapy.